1. Technical Field
The present disclosure relates to an electric vehicle including motors each configured to apply a driving torque to a corresponding drive wheel via each gear-type speed reducer.
2. Description of the Related Art
Electric vehicles, such as electric automobiles, having such a mechanism that respective drive wheels are individually driven include motors each configured to apply a driving torque to a corresponding drive wheel, and a braking apparatus configured to apply a braking torque to each drive wheel independently of one another. In this type of electric vehicle, when a travel behavior of a vehicle is controlled through control of a braking/driving torque of each drive wheel, a final target braking/driving torque is calculated for each drive wheel, which is a sum of a first target braking/driving torque that is based on a braking/driving operation amount of a driver and a second target braking/driving torque for travel behavior control. Further, each motor and a braking apparatus are controlled so that an actual braking/driving torque of each drive wheel reaches a corresponding final target braking/driving torque.
The travel behavior control is performed to control a roll motion, a pitch motion, a yaw motion, and a heave motion of the vehicle, and thus the second target braking/driving torque changes between the driving torque and the braking torque. Thus, even when the first target braking/driving torque is a driving torque (powering torque), the final target braking/driving torque may change between the driving torque and the braking torque. In contrast, even when the first target braking/driving torque is a braking torque, the final target braking/driving torque may change between the braking torque and the driving torque.
In general, in the electric vehicle having the above-mentioned mechanism, the driving torque of each motor is transmitted to a corresponding drive wheel via each gear-type speed reducer. When the final target braking/driving torque changes between the driving torque and the braking torque, a reversal of the torque direction (hereinafter referred to as “zero cross”) is caused, and the rotational direction of a gear is reversed. There is a backlash in the gear-type speed reducer, and thus teeth of engaging gears collide with each other when the rotational direction of a gear is reversed, with the result that an abnormal noise uncomfortable for occupants of the vehicle is generated.
For example, as described in Japanese Patent Application Laid-open No. 2015-77834, there is known a technology for offsetting final target braking/driving torques of front wheels and rear wheels in opposite directions in order to suppress the generation of the zero cross of a torque and its resultant generation of an abnormal sound. In this technology, offset torques in opposite directions and of the same magnitude are added to the final target braking/driving torques of the front wheels and the rear wheels, to thereby prevent the final target braking/driving torque of each wheel from changing across 0 without changing the sum of the final target braking/driving torques of the four wheels. Therefore, it is possible to prevent the generation of the zero cross and its resultant generation of an abnormal sound.
However, in the technology described in Japanese Patent Application Laid-open No. 2015-77834, the final target braking/driving torques of the front wheels and the rear wheels are offset in opposite directions. Thus, the generation of the zero cross cannot be prevented in a drive wheel that has a possibility of the generation of the zero cross through control of the final target braking/driving torque of only the drive wheel. Further, the final target braking/driving torques of the front wheels and the rear wheels are increased/decreased in opposite increasing/decreasing directions due to the addition of the offset torques, and thus an undesirable behavior change, e.g., pitching of the vehicle, may be caused due to this increase/decrease. Further, when the zero cross may be generated in both front wheels and rear wheels of a four-wheel drive vehicle, even when addition of offset torques in opposite increasing/decreasing directions is requested, offset torques cannot be added. Thus, the generation of the zero cross cannot be prevented.